The present invention relates to the magnetic resonance art. It finds particular application in conjunction with gradient coils for magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it should be appreciated that the present invention may also find application in conjunction with magnetic resonance spectroscopy systems and other applications which utilize gradient magnetic fields.
In magnetic resonance imaging (MRI) applications, three orthogonal gradient fields are employed to provide spatial resolution by frequency discrimination of an MRI signal. A gradient coil set typically includes three discrete gradient coils for generating the x, y, and z-gradient fields. The coils are insulated from each other and are layered on a cylindrical former. Commonly, the entire gradient coil set is overwrapped and epoxy impregnated for greater structural strength to withstand the warping forces when the current carrying conductors of the coils interact with the primary magnetic field of an MRI apparatus.
The gradient coils are commonly pulsed with current pulses having a short rise time and a high duty cycle. Pulsing the gradient coils produces magnetic field gradients across the imaging region, as well as magnetic field gradients which interact with external metallic structures such as cold shields in a superconducting magnet. This interaction generates eddy currents in the cold shields, which, in turn, generate eddy magnetic fields. The eddy fields have a deleterious effect on the temporal and spatial quality of the magnetic field in the examination region, hence in the resultant image quality.
One approach for circumventing the eddy current problem is to place a secondary or shielding gradient coil set between the primary gradient coil set and the cold shields. The shielding gradient coil set substantially zeroes the magnetic field externally thereof, thus preventing the formation of eddy currents in the cold shields.
A unitary, self-shielded gradient coil assembly typically includes a secondary or shielding gradient coil set spaced radially outwardly of a primary gradient coil set and driven in series therewith. The primary and secondary gradient coil sets include insulated coils for generating x, y, and z-gradient fields which are layered on separate cylindrical formers. The primary and secondary gradient coil sets are individually overwrapped and epoxy impregnated for greater structural strength. A mechanical means typically connects the two formers together to form a unitary structure while maintaining the coil sets in a spaced relationship.
Typically, main superconducting magnets have bore diameters of least 90 cm and are at least 1.8 m in length. Magnets of such size can accommodate shielded gradient coil structures that are constructed from two separate formers (wherein each former has a thickness of approximately 5.0 to 7.5 cm) because there is adequate space between the predetermined patient aperture and the inside bore diameter of the magnet to accommodate such a radial build.
Reducing the diameter of the magnet bore dramatically reduces magnet costs. However, reducing the patient receiving bore reduces the utility of the MRI apparatus and its commercial acceptability.
In addition, known methods of manufacturing self-shielded gradient coil assemblies have a time-consuming, final alignment step. Alignment tooling and fixtures are used to ensure that the axial isocenter of the primary gradient coils coincide with the axial isocenter of the secondary gradient coils.
The present invention contemplates a new and improved self-shielded gradient coil assembly and method for manufacturing such a self-shielded gradient coil assembly which overcomes the above-referenced problems and others.